It is known that the initial segment of strip, produced in non-stationary conditions, has an irregular conformation which could make it difficult for the strip to enter the pinch-roll or possible rolling stands, which usually have a limited aperture.
This irregular conformation, moreover, has a quality which is unacceptable from the production point of view.
The final segment of the cast strip is also produced in non-stationary conditions and therefore has an unacceptable quality.
The solution to this problem is to remove a leading or trailing end segment, of the desired length, by means of shearing equipment located immediately downstream of the continuous casting machine and upstream of the first rolling stand.
However, this solution, especially for steel, has technical and operating problems which often make it impossible to use. In fact, the environment downstream of the continuous casting machine is very critical, particularly because the strip, downstream of the casting rollers, is made to pass inside a closed environment with an inert atmosphere and kept at high temperature, in the case of steel around 700÷1000° C., to prevent surface oxidation of the strip. This entails considerable difficulties in installing and controlling shearing equipment such as shears or similar, and also in their functioning.
The need to interrupt a strip cleanly while it is being cast can occur even in an intermediate step of the casting, for example in emergency situations, or to separate the production of strips of different thickness, or again, for production reasons, to divide the strip into several segments during a casting in stationary conditions.
Documents U.S. Pat. No. 5,690,163 and U.S. Pat. No. 5,287,912 disclose a method to shear a strip during the casting step by means of increasing, for a brief period, the distance between the casting rollers, keeping the casting speed constant, said increase being then followed by a restoration of the distance adopted in the stationary regime. This increase in thickness of the strip, maintaining the same casting speed, and hence maintaining the same time of contact between the strip and the cooled rollers, determines the formation of an inclusion of liquid metal between two thicknesses of solidified skin. This inclusion causes, downstream of the kissing point, the skins to melt again and the strip to break due to its own weight.
The present Applicant has devised and embodied this invention to overcome the shortcomings of the state of the art and to obtain other advantages as shown hereafter.